The present invention relates to a coupling for coupling together two mutually coaxial, rotatable parts, e.g. a shaft and a hub, comprising a substantially cylindrical coupling member which is intended to be positioned between the parts to be coupled together and which is expandable both radially outwards and radially inwards so as to exert on said parts a coupling pressure through the intermediary of the coupling member and optionally through the intermediary of intermediate members, and in which the coupling member incorporates a pressure chamber which extends along said member and which is intended to be supplied with a pressure fluid to effect said expansion of the coupling member and which is provided with a pressure relief device which is triggered upon relative rotation of said parts.
Such a coupling is known to the art from UK Patent Specification No. 2 023 250, for instance. In the case of this known coupling, a shaft or axle is coupled to a hub with the aid of a radially inwards and outwards expandable, cylindrical coupling member, wherewith it is ensured that the coupling will exert a greater coupling force between itself and the hub than between itself and the shaft, among other things by mounting an intermediate sleeve between the outside of the shaft and the inside of the coupling member. In the event of an overload, the shaft will therefore rotate relative to the coupling and the hub mounted thereon, whereupon a pressure-relief device operative between the shaft and the coupling is triggered and causes the hub, coupling and shaft to disengage one from the other.
In some transmission systems, torque peaks of very short duration will occur, e.g. durations typically in the order of 1-100 milliseconds. Typical machines which include such transmission systems are electrical generators driven by wind power, diesel engines and gas-turbines. There are two reasons for the occurrence of extremely high torque peaks of short duration in such machines. One of these reasons is the occurrence of short circuits in the electrical system and the other is the result of faulty in-phasing on an electric network. Other types of operation in which such high torque peaks can occur are the use of frequency-controlled inductance motors to achieve high power outputs, e.g. for driving high speed locomotives and rolling mills.
The techniques used at present to overcome problems of high torques of short duration (torques which are ten times the normal torque are common) involve:
1. Overdimensioning the transmission system, so that the system is able to handle the torque peaks. The drawback with this solution, however, are that overdimensioning is expensive and results in a system of undue weight.
2. The use of shear-pin couplings or like devices. The drawback here is that it is necessary to re-establish couplings manually, which results in non-productive downtime periods, which can be very expensive, particularly in the case of unmanned systems.
3. The use of disc-type friction couplings. The drawback here is the very large coupling diameters required and the resultant large flywheel mass, which in the majority of cases renders this type of solution impracticable.